1. Field of the Invention
The invention relates to an apparatus for measuring the mobility of dispersed particles in a liquid medium, from which the zeta potentials of those particles may be calculated.
2. Description of the Prior Art
The term "zeta potential" is understood in colloidal chemistry as an indicator of the electrophoretic mobility of particulate matter colloidally suspended or dispersed in a fluid medium. The electrophoretic mobility, in turn, is an indication of the velocity of the particles through the solution due to the effect of an applied electric field.
With regard to colloidal particles suspended in a fluid medium, the suspended particles are considered electrically neutral. That is, each particle is surrounded by a double layer of electrical or electrostatic charges, the inner layer of which is immediately adjacent to and fixed to the particle, while the outer layer, which possesses a charge opposite to that of the inner layer, is attracted by the surface charge and extends out into the solution for some distance.
The zeta potential, therefore, is not measured on the surface of a particle but rather at what is known as the "slipping" or "shear" plane where the net difference in force between the particle and the bulk solution which surrounds it is small enough so that the particle may move independently within an electrophoretic cell when an electric potential is applied across the electrodes of the cell. This migration occurs because the charged groups and ions adsorbed on the surface of the colloidal particles produce a non-uniform distribution of ions in solution near the particle-liquid interface.
The stability of a particle suspended in a bulk medium is directly related to the zeta potential of the particle. Stable particles remain dispersed whereas unstable particles tend to agglomerate and eventually precipitate out of the solution. The higher the zeta potential, the more stable the system is since highly charged particles repel one another and remain dispersed.
In some cases, such as the manufacture of paint, pharmaceuticals and cosmetics, it is desirable to maximize the zeta potential so as to achieve greater stability. Conversely, in situations involving the treatment of waste water or during the manufacture of paper, it is important to minimize this potential, as this leads to the agglomeration of the colloidal particles and the settling out of the flocculate formed thereby.
Various means have been utilized to measure the zeta potentials of colloidal particles suspended in a liquid medium. U.S. Pat. No. 3,454,487 to Riddick discloses a manually operated microelectrophoresis apparatus, wherein the electrophoretic mobility, i.e. the velocity of the particles per unit field strength, is measured in an electrophoretic cell which consists of sample receiving chambers connected by a liquid-flow communication passageway.
This measurement is performed utilizing an ocular micrometer or distance scale which is inserted into the eyepiece of a microscope. The particles are timed as they cross a fixed distance in the observation chamber of the electrophoretic cell under a D.C. electric field of known strength. This method is time consuming and tedious to employ since repeated operations are necessary to accurately calibrate the instrument and determine the zeta potential in a sample bulk medium.
In order to facilitate more rapid and efficient measurements of electrophoretic mobility, attempts were made to develop a "semi-automatic" microelectrophoresis apparatus as disclosed in U.S. Pat. No. 3,764,512 to Greenwood et al. In this apparatus, a coherent light beam from a laser is caused to intermittently scan a path located on the stationary layer of an electrophoresis chamber by means of a mirror galvanometer at a rate equal to the migration rate of the particles in the chamber. The operator views the migrating particles in the chamber through a microscope and simultaneously adjusts the scanning rate of the mirror galvanometer by adjusting a potentiometer in the galvanometer control circuit until the scanning laser beam appears to visually track the migrating particles as viewed through the microscope. Via appropriate scaling circuitry interacting with the galvanometer drive circuit and the circuit supplying the voltage drop across the chamber, a value for electrophoretic mobility or zeta potential may automatically be displayed through suitable means, such as an electronically operated digital readout.
U.S. Pat. No. 3,793,180 to Flower et al. discloses a fully automated system capable of measuring zeta potential, particle size distribution, total charge density and other distribution functions of aqueous suspensions. A laser beam is provided in the instrument and is focused on the particles in the sample solution which are contained in an electrophoretic cell. A reticle or grating is positioned such that the reflection of the laser beam from the particles passes through the reticle to a photo-tube placed on the other side of the reticle. Then, as the particles move through the solution the photo-tube is intermittently illuminated through the reticle and, as a result, generates a train of electric pulses whose frequency is directly proportional to the velocity of the particles. Since the frequency of the electrical signal is a measurement of the particle velocity, it is also a measure of zeta potential. The advantage of a fully automated system is that it serves to reduce the human error associated with the manual operation of the prior methods.
In U.S. Pat. No. 3,909,380 to Day, a television camera or other equivalent image sensor, such as a photosensitive array utilizing a suitable lens system, observes the fluid within an electrophoresis cell which is illuminated by the cold light of a fiberoptic source in order to prevent the production of convection currents in the medium.
The suspended particles are magnified by a microscope and the image is projected onto the monitor screen. A reference pattern is superimposed onto the monitor and the sweep speed of the reference pattern is then manually adjusted to match the speed of any single particle or group of particles on the monitor screen. The sweep speed of the reference pattern may then be converted to a zeta potential signal which must be corrected for the temperature of the sample.
Finally, U.S. Pat. No. 4,046,667 to Goetz describes an electrophoresis chamber, a circuit for impressing a voltage across the chamber, a light beam to illuminate a portion of the chamber, and a microscope including an objective lens system and an eyepiece for viewing illuminated particles migrating relative to a suspending medium within the chamber under the influence of the applied voltage. Within the microscope, between the objective lens and the eyepiece, is a movable optical prism driven by a galvanometer, the drive circuit of which includes an adjustable potentiometer for controlling the rate and direction of movement of the optical prism.
A circuit connected to the galvanometer drive circuit and the circuit applying the voltage potential across the chamber are adapted to develop a signal proportional to the electrophoretic mobility or zeta potential of the migrating particles when the rate of movement of the optical prism is adjusted such that it cancels the transfer velocity of the migrating particles. The particles then appear stationary when observed through the eyepiece of the microscope. Because the movable optical prism is located inside of the microscope, between the objective lens and the eyepiece, it is possible to employ an electrophoresis chamber having a rectangular cross-sectional shape such that its height is significantly less than its width. By virtue of this geometry the electrophoresis chamber is rendered relatively impervious to thermodynamic effects. The electrophoresis chamber, which is constructed of three relatively thin flat plates, also includes means for mounting and supporting a pair of spaced electrodes; as well as means for avoiding entrapment of air bubbles while being filled with a sample.
Applicants have now discovered an improved apparatus and method for measuring the mobility of colloidal particles suspended in a liquid medium which avoids the drawbacks of the prior art.